feat(api): extend commit and diff services with new functionality

- Add FindCommit, ListCommitsByOid, CommitIsAncestor RPCs to CommitService
- Add CheckObjectsExist, CommitsByMessage, GetCommitStats RPCs to CommitService
- Add LastCommitForPath, CountCommits, CountDivergingCommits RPCs to CommitService
- Add RawDiff, RawPatch, FindChangedPaths RPCs to DiffService
- Add FindMergeBase, WriteRef, SearchFilesByContent RPCs to RepositoryService
- Add SearchFilesByName, ObjectsSize, RepositorySize RPCs to RepositoryService
- Add FindLicense, OptimizeRepository, GetRawChanges RPCs to RepositoryService
- Add FetchRemote, CreateRepositoryFromURL RPCs to RepositoryService
- Implement server handlers for all new RPC methods
- Add new modules for commit counting, finding, and querying features
- Add new modules for diff changed paths and raw operations
- Add new modules for refs and remote operations
- Remove unnecessary comments from various source files
- Update proto definitions with new message types and service methods

proto/ref.proto

@@ -0,0 +1,99 @@
+syntax = "proto3";
+
+package gitks;
+
+import "google/protobuf/empty.proto";
+import "oid.proto";
+import "repository.proto";
+
+
+message FindDefaultBranchNameRequest {
+  RepositoryHeader repository = 1;
+}
+
+message FindDefaultBranchNameResponse {
+  string name = 1;
+}
+
+message RefExistsRequest {
+  RepositoryHeader repository = 1;
+  string ref_name = 2;
+}
+
+message RefExistsResponse {
+  bool exists = 1;
+}
+
+
+message RefUpdateEntry {
+  string ref_name = 1;
+  string new_oid = 2;
+  string old_oid = 3; // expected old OID (empty = no check)
+}
+
+message UpdateReferencesRequest {
+  RepositoryHeader repository = 1;
+  repeated RefUpdateEntry updates = 2;
+}
+
+message UpdateReferencesResponse {
+  repeated string failed_refs = 1;
+  string error = 2;
+}
+
+message DeleteRefsRequest {
+  RepositoryHeader repository = 1;
+  repeated string ref_names = 2;
+}
+
+message DeleteRefsResponse {
+  repeated string failed_refs = 1;
+  string error = 2;
+}
+
+
+message FindRefsByOIDRequest {
+  RepositoryHeader repository = 1;
+  string oid = 2;
+  RefFilter filter = 3;
+}
+
+message RefFilter {
+  repeated string prefixes = 1; // e.g. ["refs/heads/", "refs/tags/"]
+  uint32 limit = 2;
+}
+
+message FoundRef {
+  string ref_name = 1;
+  string target_oid = 2;
+  bool symbolic = 3;
+  string symbolic_target = 4;
+}
+
+message FindRefsByOIDResponse {
+  repeated FoundRef refs = 1;
+}
+
+
+message ListRefsRequest {
+  RepositoryHeader repository = 1;
+  repeated string prefixes = 2;
+  string pattern = 3;    // glob pattern, e.g. "refs/heads/*"
+  repeated string containing_oids = 4;
+  SortDirection sort_direction = 5;
+  Pagination pagination = 6;
+}
+
+message ListRefsResponse {
+  repeated FoundRef refs = 1;
+  PageInfo page_info = 2;
+}
+
+service RefService {
+  rpc FindDefaultBranchName(FindDefaultBranchNameRequest) returns (FindDefaultBranchNameResponse);
+  rpc RefExists(RefExistsRequest) returns (RefExistsResponse);
+  rpc UpdateReferences(UpdateReferencesRequest) returns (UpdateReferencesResponse);
+  rpc DeleteRefs(DeleteRefsRequest) returns (DeleteRefsResponse);
+  rpc FindRefsByOID(FindRefsByOIDRequest) returns (FindRefsByOIDResponse);
+  rpc ListRefs(ListRefsRequest) returns (ListRefsResponse);
+}